Cross-Compiling from x86_64 to ARM

In this use case, we'll be using CMake and Ninja, on a Debian-based Linux
system, cross-compiling from an x86_64 host (most Intel and AMD chips
nowadays) to a hard-float ARM target (most ARM targets nowadays).

The packages you'll need are:

cmake

ninja-build (from backports in Ubuntu)

gcc-4.7-arm-linux-gnueabihf

gcc-4.7-multilib-arm-linux-gnueabihf

binutils-arm-linux-gnueabihf

libgcc1-armhf-cross

libsfgcc1-armhf-cross

libstdc++6-armhf-cross

libstdc++6-4.7-dev-armhf-cross

Configuring CMake

For more information on how to configure CMake for LLVM/Clang,
see :doc:`CMake`.

The TableGen options are required to compile it with the host compiler,
so you'll need to compile LLVM (or at least llvm-tblgen) to your host
platform before you start. The CXX flags define the target, cpu (which
defaults to fpu=VFP3 with NEON), and forcing the hard-float ABI. If you're
using Clang as a cross-compiler, you will also have to set -ccc-gcc-name,
to make sure it picks the correct linker.

Most of the time, what you want is to have a native compiler to the
platform itself, but not others. It might not even be feasible to
produce x86 binaries from ARM targets, so there's no point in compiling
all back-ends. For that reason, you should also set the
TARGETS_TO_BUILD to only build the ARM back-end.

You must set the CMAKE_INSTALL_PREFIX, otherwise a ninja install
will copy ARM binaries to your root filesystem, which is not what you
want.

Hacks

There are some bugs in current LLVM, which require some fiddling before
running CMake:

If you're using Clang as the cross-compiler, there is a problem in
the LLVM ARM back-end that is producing absolute relocations on
position-independent code (R_ARM_THM_MOVW_ABS_NC), so for now, you
should disable PIC:

-DLLVM_ENABLE_PIC=False

This is not a problem, since Clang/LLVM libraries are statically
linked anyway, it shouldn't affect much.

The ARM libraries won't be installed in your system, and possibly
not easily installable anyway, so you'll have to build/download
them separately. But the CMake prepare step, which checks for
dependencies, will check the host libraries, not the target
ones.

A quick way of getting the libraries is to download them from
a distribution repository, like Debian (http://packages.debian.org/wheezy/),
and download the missing libraries. Note that the libXXX
will have the shared objects (.so) and the libXXX-dev will
give you the headers and the static (.a) library. Just in
case, download both.

The ones you need for ARM are: libtinfo, zlib1g,
libxml2 and liblzma. In the Debian repository you'll
find downloads for all architectures.

After you download and unpack all .deb packages, copy all
.so and .a to a directory, make the appropriate
symbolic links (if necessary), and add the relevant -L
and -I paths to -DCMAKE_CXX_FLAGS above.

Running CMake and Building

Finally, if you're using your platform compiler, run:

$ cmake -G Ninja <source-dir> <options above>

If you're using Clang as the cross-compiler, run:

$ CC='clang'CXX='clang++' cmake -G Ninja <source-dir> <options above>

If you have clang/clang++ on the path, it should just work, and special
Ninja files will be created in the build directory. I strongly suggest
you to run cmake on a separate build directory, not inside the
source tree.

To build, simply type:

$ ninja

It should automatically find out how many cores you have, what are
the rules that needs building and will build the whole thing.

You can't run ninja check-all on this tree because the created
binaries are targeted to ARM, not x86_64.

Installing and Using

After the LLVM/Clang has built successfully, you should install it
via:

$ ninja install

which will create a sysroot on the install-dir. You can then tar
that directory into a binary with the full triple name (for easy
identification), like:

If you copy that tarball to your target board, you'll be able to use
it for running the test-suite, for example. Follow the guidelines at
http://llvm.org/docs/lnt/quickstart.html, unpack the tarball in the
test directory, and use options: